The adaptive value of dental crowding: a consideration of the biologic basis of malocclusion.

Malocclusion, an ambiguous concept that refers to structural disharmony of the teeth and jaws, can be defined only in reference to normal occlusion. If normal occlusion is synonymous only with ideal occlusion, then normal occlusion becomes a rare occurrence. A biologically valid concept of normal occlusion includes a range of variation in the relevant occlusal variables that is compatible with health and unimpaired function. The difficulty in defining malocclusion is the determination of the point at which normal variation becomes abnormal. A clearer concept of malocclusion is obtained if the occlusal variables that it comprises are considered. Dental crowding is endemic among technologically advanced populations and uncommon in primitive groups. The significant elements in the development of most dental crowding are mesial migration and the lack of interproximal attrition. Mesial migration of the posterior teeth provides the functional replacement for the tooth surface lost to attrition because of the rigors of a primitive diet. In modern man there is little attrition of the teeth because of a soft, processed diet; this can result in dental crowding and impaction of the third molars. It is postulated that the tooth-jaw size discrepancy apparent in modern man as dental crowding is, in primitive man, a crucial biologic adaptation imposed by the selection pressures of a demanding diet that maintains sufficient chewing surface area for long-term survival. Selection pressures for teeth large enough to withstand a rigorous diet have been relaxed only recently in advanced populations, and the slow pace of evolutionary change has not yet brought the teeth and jaws into harmonious relationship.